Among air conditioning apparatus, there is proposed one equipped with an outdoor unit, a relay unit, and indoor units such as in a multi air conditioning system for a building, in which the outdoor unit and the relay unit are connected by refrigerant pipes in which refrigerant circulates, and in which the relay unit and the indoor units are connected by heat medium pipes in which a heat medium circulates (see Patent Literature 1, for example). With the technology described in Patent Literature 1, the outdoor unit and the indoor units are connected via a relay unit including an intermediate heat exchanger that induces heat exchange between refrigerant and heat medium, thereby enabling a reduction in the transport power of the refrigerant as well as the transport performance of the heat medium. Also, with the technology described in Patent Literature 1, the relay unit includes multiple intermediate heat exchangers and multiple flow switching devices, thereby enabling cooling and heating mixed operation to be carried out.
Also, there is proposed a refrigeration device that, in order to lower the discharge temperature of a compressor and thereby cause the compressor to operate stably, irrespective of a refrigerant circuit, operating state, or the like, a refrigerant pipe carrying high-pressure liquid refrigerant is connected to an intermediate pressure unit of the compressor, and liquid injection into the compressor is conducted (see Patent Literature 2, for example).
Furthermore, there is proposed an air conditioning device that includes a refrigerant circuit in which a check valve is connected in parallel with an expansion device provided on the indoor side, and in addition, a check valve is also connected in parallel with an expansion device on the outdoor side (see Patent Literature 3, for example). With this refrigerant circuit, the technology described in Patent Literature 3 enables high-pressure liquid refrigerant to be supplied to a pipe connecting the suction side of the compressor to an accumulator, and injected into the compressor, even if the flow of refrigerant changes due to switching between cooling operation and heating operation.